Chapter 8 Project Scheduling

INTRODUCTION

Schedule converts action plan into operating

time tableBasis for monitoring and controlling projectScheduling more important in projects thanin production, because unique natureSometimes customer specified/approvedrequirement-e.g: JKR projectsBased on Work Breakdown Structure (WBS)

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NETWORK TECHNIQUESPERT

CPM

-Program Evaluation and

Review Technique- developed by the USNavy with BoozHamilton Lockheed- on the PolarisMissile/Submarineprogram 1958

Critical Path Method

Developed by ElDupont for ChemicalPlant ShutdownProject- aboutsame time as PERT

Both use same calculations, almost similar

Main difference is probabilistic and deterministic in timeestimationGantt Chart also used in scheduling

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NETWORK

Graphical portrayal of activities and event

Shows dependency relationships betweentasks/activities in a projectClearly shows tasks that must precede(precedence) or follow (succeeding) othertasks in a logical mannerClear representation of plan a powerfultool for planning and controlling project

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Example of Simple Network

Survey

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Example of Network More Complex

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DEFINITION OF TERMS IN A NETWORK

Activity:any portions of project (tasks) which required byproject, uses up resource and consumestime may involve labor,paper work,contractual negotiations, machinery operationsActivity on Arrow (AOA) showed as arrow, AON Activity on NodeEvent:beginning or ending points of one or moreactivities, instantaneous point in time, alsocalled nodesNetwork

Combination of all project activities and the events

SUCCESSOR

PRECEEDINGACTIVITY

EVENT

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Emphasis on Logic in Network Construction

Construction of network should be based on logical or

technical dependencies among activitiesExample - before activity Approve Drawing can bestarted the activity Prepare Drawing must be completedCommon error build network on the basis of time logic(a feeling for proper sequence ) see example below

WRONG !!!Chapter 8

CORRECT Scheduling, PERT, Cri

Example 1- A simple network

Consider the list of four activities for making a simpleproduct:Activity

Description

Buy Plastic Body

Design Component

Make Component

Assemble product

ImmediatepredecessorsBA,C

Immediate predecessors for a particular activity are

the activities that, when completed, enable the start ofthe activity in question.

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Sequence of activities

Can start work on activities A and B anytime,

since neither of these activities depends uponthe completion of prior activities.

Activity C cannot be started until activity B has

been completed

Activity D cannot be started until both

activities A and C have been completed.

The graphical representation (next slide) is

referred to as the PERT/CPM network

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Network of Four Activities

Arcs indicate project activitiesA

2Nodes correspond to thebeginning and ending ofactivities

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Example 2Develop the network for a project with following activitiesand immediate predecessors:ActivityImmediatepredecessorsAB

A, C

Try to do forG the first fiveD,E,F(A,B,C,D,E) activitiesChapter 8

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Network of first five activities

A

BC

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5We need tointroduce adummy activity

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Network of Seven Activities

A

3dummy

BC

EF

Note how the network correctly identifies D, E, and F as the

immediate predecessors for activity G.Dummy activities is used to identify precedencerelationships correctly and to eliminate possible confusion oftwo or more activities having the same starting and endingnodesDummy activities have no resources (time, labor, machinery,etc) purpose is to PRESERVE LOGIC of the network

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EXAMPLES OF THE USE OF DUMMYACTIVITY

Network concurrent activitiesa

a2

Dummy

WRONG!!!

RIGHT

Activity c notrequired for e

WRONG !a

ab

d1

cc

WRONG!!!

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RIGHT

15

WRONG!!!

RIGHT!!!

a precedes d.a and b precede e,b and c precede f (a does not precede f)

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Scheduling with activity time

Activity(week)ABCDEFGHI

ImmediatepredecessorsAAAED,FB,CG,H

CompletionTime

Total

5643141412251

This information indicates that the total time required to

complete activities is 51 weeks. However, we can see from the

network that several of the activities can be conductedsimultaneously (A and B, for example).

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Earliest start & earliest finish time

We are interested in the longest path through the

network, i.e., the critical path.

Starting at the networks origin (node 1) and using

a starting time of 0, we compute an earlieststart (ES) and earliest finish (EF) time for eachactivity in the network.

The expression EF = ES + t can be used to find

the earliest finish time for a given activity.For example, for activity A, ES = 0 and t = 5; thusthe earliest finish time for activity A isEF = 0 + 5 = 5

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Arc with ES & EF time

EF = earliest finishtime

ES = earliest starttimeActivity

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]5,0[A5

t = expected activitytimeScheduling, PERT, Cri

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Network with ES & EF time

D[5,8]3

0]16,[F4

B[0,6]6

, 21]9[H12

54]0,2G[1 41

E[5,6]1

C[5,9]4

A[05 ,5]

26],42I[2

3Earliest start time rule:The earliest start time for an activity leaving a particular nodeis equal to the largest of the earliest finish times for allactivities entering the node.

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Activity, duration, ES, EF, LS, LF

ES = earliest starttimeActivity]9,5[C2]1,8[4

LS = latest starttimeChapter 8

EF = earliest finishtime

LF = latest finishtime

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Latest start & latest finish time

To find the critical path we need a backward pass calculation.

Starting at the completion point (node 7) and using a latest

finish time (LF) of 26 for activity I, we trace back through thenetwork computing a latest start (LS) and latest finish timefor each activity

The expression LS = LF t can be used to calculate latest start

time for each activity. For example, for activity I, LF = 26 and t= 2, thus the latest start time for activity I isLS = 26 2 = 24

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Network with LS & LF time

A[5[ 0,5]0,5]B[06[6 ,6], 12]

C[5,9]4[8,12]

E[1[5 5,6],6]

0]16, 10][F 6,4[

21],9[H,24]21[12

54]0,2 ]G[1 10,2414[

D[5,8]3[7,10]

26],42I[26],422[

3Latest finish time rule:

The latest finish time for an activity entering a particular node is

equal to the smallest of the latest start times for all activitiesleaving the node.

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Slack or Free Time or Float

Slack is the length of time an activity can be delayed withoutaffecting the completion date for the entire project.For example, slack for C = 3 weeks, i.e Activity C can be delayedup to 3 weeks3(start anywhere between weeks 5 and 8). [5,9]

2ES5

LS8

EF9

C2]1,8[4

EF12LF-EF = 12 9 =3LS-ES = 8 5 = 3

LF-ES-t = 12-5-4 = 3

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Activity schedule for our example

Activity

ABCDEFGHI

Earlieststart (ES)

Lateststart (LS)

00555610924

068756101224

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Earliestfinish(EF)

5698610242126

Latestfinish(LF)

5121210610242426

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Slack(LS-ES)

063200030

Criticalpath

Yes

YesYesYesYes25

IMPORTANT QUESTIONS

What is the total time to complete the project?

What are the scheduled start and completion times for eachactivity?

ES, EF, LS, LF are given for each activity.

What activities are critical and must be completed as

scheduled in order to keep the project on time?

26 weeks if the individual activities are completed on schedule.

Critical path activities: A, E, F, G, and I.

How long can non-critical activities be delayed before they

cause a delay in the projects completion time

Slack time available for all activities are given.

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Importance of Float (Slack) and Critical Path

1.

Slack or Float shows how much allowance each activity has,

i.e how long it can be delayed without affecting completiondate of project

2.

Critical path is a sequence of activities from start to finish

with zero slack. Critical activities are activities on the criticalpath.

3.

Critical path identifies the minimum time to complete

project

4.

If any activity on the critical path is shortened or extended,

project time will be shortened or extended accordingly

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Importance of Float (Slack) and Critical Path (cont)

5.

So, a lot of effort should be put in trying to control activities

along this path, so that project can meet due date. If any activityis lengthened, be aware that project will not meet deadline andsome action needs to be taken.